Road traffic control system

ABSTRACT

This invention provides an apparatus for effecting optimal road traffic control of a road network in a traffic area, such a traffic area being divided into a plurality of sub-areas and a district controller being provided in each sub-area. The district controller is provided to control the traffic in the sub-area, road network, establishing a preferential offset for two-way traffic for a tree pattern of the road network which includes no closed road loop, by considering the offset effect quantum for each road section between signals, and a split for each intersection and a common cycle length thereby minimizing the delay time in the sub-area as based on information from traffic detectors on the road and other road conditions. A Central Controller is provided for controlling traffic of the entire road network in the area by coordinating the district controllers as based upon the traffic pattern of the entire area.

United States Patent Inose et al. [4 1 May 2, 1972 ROAD TRAFFIC CONTROL SYSTEM 721 Inventors: Hlroshi Inose; Hiroya Fujisaki; Takashi [57] ABSTRACT Ramada, all of Tokyo, Japan This invention provides an apparatus for effecting optimal road traffic control of a road network in a traffic area, such a [73] Asslgnee' i r t Electric Industries Osaka trafiic area being divided into a plurality of sub-areas and a P district controller being provided in each sub-area. The dis- [22] Filed: May 1, 1970 trict controller is provided to control the traffic in the subarea, road network, establishing a preferential offset for two- [211 App]? 33826 way traffic for a tree pattern of the road network which in- Rehed us. Application Datacludes no closed road loop, by considering the offset effect I quantum for each road section between signals, and a split for Continuation-Invert 0f N0; P each intersection and a common cycle length thereby 1967, abandonedminimizing the delay time in the sub-area as based on information from traffic detectors on the road and other road condi- [52] US. Cl ..340/35 ti A Cent l Controller is provided for controlling traffic 8 1/08 of the entire road network in the area by coordinating the dis- [58] Field Of Search ..340/35 trict t ll 35 bas d pon the traffic pattern of the entire Primary Examiner-William C. Cooper Attorney-Carothers and Carothers area.

9 Claims, 32 Drawing Figures PATENTEDMAY 2 I972 3, 660, 8 1 2 RF Olsen/mu; 70R

I 2302 I I g 0- L 2373 2392 l i luvzlvroks l-llfiosm I NO $5, I H120 YA FuwsAxui TAKASH/HAMADA 5v CAEOTHER$AEO 11/525 THE/E A rraeuevs ROAD TRAFFIC CONTROL SYSTEM CROSS REFERENCE This application is a continuation-in-part of application Ser. No. 631,056, filed Apr. 14, 1967, for Road Traffic Control System, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a road traffic control system, and more particularly to a system for controlling traffic signals in a large urban area such as a large city.

It is already known to those skilled in the art that a smooth flow of traffic in a signalized road network is effected by assigning appropriate offsets to each signalized intersection. Signal control systems for artery road networks are well known, and are employed in many cities. Also, a grid network is well known and is disclosed in [1.8. Pat. No. 3,120,651 by G. D. Hendricks wherein eastbound and westbound traffic volumes and northbound and southbound traffic volumes are compared by special purpose computers and preferential offset is assigned to the direction of the higher trafiic volume of either the east-west streets or the north-south streets.

These techniques, however, cannot be applied directly to large scale and complicated road networks, such as a grid network cut by diagonal streets, or in a situation of a no-grid network, in which the sequence of preferential offsets cannot be assigned to the streets because of the complexity of the traffic pattern and the ambiguity of the higher traffic directions. In these cities, the optimal value of offset for a road section (i.e., a portion of road between two adjacent signalized intersections) should be determined in accordance with the traffic conditions of each of the road sections.

These techniques also cannot be applied directly to a complicated road network which contains closed loops of road sections, because the determined values of offset cannot be given consistently to all of the road sections which comprise a closed loop, and any one of the ofisets for these road sections will be fixed when offsets are given to all of the remaining road sections in the loop. Therefore, a set of road sections in the network to which an arbitrary value of offset can be given are restricted to a subset of road sections which contains no closed loop. A set of connected road sections within a network, which contains no closed loop and in which all of the nodes (i.e. intersections) are connected by links (i.e. road sections) is topologically called a tree or tree pattern" of the original network. Therefore, the road sections to which an arbitrary offset can be given are restricted to the ones which are included in a tree pattern. 1

The optimal group or set of offsets for the whole network are established by selecting an optimal tree pattern out of a number of possible tree patterns and by assigning appropriate offsets to each of the road sections constituting the optimal tree pattern. However, it is difficult to determine this optimal tree pattern because there are a great number of tree patterns in a road network.

In the prior art, the optimal set of offsets for complicated road networks was determined by the simulation technique using electronic digital computers or by an empirical method. However, for a large-scale road network, the simulation technique requires a great 'dealof calculation time, so that such a technique is not applicable to the on-line control system. On the other hand, the empirical method is not expected to result in effective offset values for a complicated road network.

To make the flow of traffic smooth, means are required to minimize as far as possible the total waiting time of traffic at signals and to eliminate the congestion of traffic flow.

As traffic signal control systems to attain this object, there has heretofore been in use a system for controlling a plurality of signals installed at only one intersection (the so-called point control), and a system for controlling all together, a plurality of signals installed along a road (the so-called artery control). A system for controlling signals existing in a generally two-dimentional road network (the so-called area control) has also been composed by installing in combination the afore-mentioned control devices which are capable of point control or artery control.

It goes without saying that a signal control system in a city that has a complicated road network must be one capable of area control. F or such a system, however, an overall control of all the signals is necessary and for this purpose, it is necessary to pay full consideration to the composition of the control system and control area. The afore-mentioned point control system is, in itself, a system for controlling one intersection independently and it is difficult to produce an overall control system with it. The production of an overall control system through a combination of artery control systems is accompanied by great difiiculty when a complicated road network as seen in large cities is to be controlled, and a control system of such a composition has not been realized yet.

The traffic signal control system of the present invention is aimed at controlling a complicated road network and is applicable to any type of a road network. The superiority of this system to the systems heretofore in use will be made clear by the following explanation.

SUMMARY OF THE INVENTION The traffic signal control system of the present invention includes a plurality of traffic detectors for each road section of a complex road network pattern in order to detect traffic flow. The output of the detectors is processed by a traffic counter device for producing signal information of traffic volume and density. A cycle selecting device responding to this signal information is provided to select an optimal traffic signal cycle length and a split selecting device responds to the signal information to determine the traffic signal split value at each signalized intersection. The control system of the present invention is uniquely provided with an offset selecting device which includes a traffic pattern detecting device, a tree selecting device, and a unit offset selecting device to determine an optimal tree pattern of the road network in response to the aforesaid traffic information on volume and density, and assigns an optimal offset to each road section included in the selected tree.

The optimal tree may be determined by the offset selecting device on the basis ofon-the-line computing by a gate network which has the same pattern as the road network and assigns the preferential or balanced ofi'set to each road section included in the determined tree.

Alternatively, the offset selecting device may select the optimal tree from a plurality of tree patterns which are predetermined in accordance with the standardized traffic patterns of the road network in question, and which thereafter assigns the preferential or balanced offset to each road section included in the selected tree.

The traffic control system may be for a large traffic control area which is divided into a plurality of districts, each of which is controlled by a district controller as just previously described. In this event, the control system includes a central controller which in turn includes a traffic pattern detector which receives the traffic information transmitted from the district controllers to determine the traffic flow pattern of the area. A switching network is also provided in the central controller which, responding to the flow pattern, selects at least one tree pattern of the area from predetermined sets of tree patterns and coordinates the district controllers of each district belonging to each selected tree pattern by assigning a cycle value common to each group of districts included in a tree pattern and by assigning optimal offset values to road sections which connect the adjacent districts.

The line computing technique of determining the optimal tree may be described as a traffic signal offset determination circuit which comprises an offset effect circuit means to calculate the values of offset effect, which is defined for each road section as the difference between the average and minimum values of traffic delay time in both. directions, for all of the road sections, together with a tree selection circuit means which successively selects the road sections having the largest offset effect value and rejects those road sections which would form a closed loop with previously selected road sections of higher offset effect value or quantum until all of the road sections have been accordingly considered such that a connected this invention wherein:

FIG. 1 is a diagrammatic plan view showing the form of an example of a road network to be controlled by the system of this invention, the district controllers (DC) and central controller (CC) installed in that road network command circuits from said central controller to said district controllers and the control districts of said district controllers.

FIG. 2 is a block diagram showing the central controller, district controller, local controller, traffic flow detection device and information transmission between said devices.

FIG. 3 is a diagrammatic plan view showing a part of a road network involving two intersections.

FIGS. 4a to 4d graphically show signal indications of the four traffic signals installed at the two intersections shown in FIG. 3.

FIG. 5 is -a flow chart for the determination of a cycle length.

FIG. 6 is a flow chart for the determination of a split.

FIG. 7 is a flow chart for the determination of a relative offset.

FIGS. 8a and 8b are diagrammatic plan views showing examples'of two trees for the district forming a part of the road network shown in FIG. 1.

F 16.9 is a flow chart for the determination of a tree.

FIG. 10 is a block diagram showing the construction of a traffic flow detection device.

FIG. 11 isa block diagram showing the construction of a local signal controller.

FIG. 12 is a block diagram showing the construction of a district controller.

FIG. 13 is a graphic illustration showing the procedure of determining a cycle length with a district controller in accordance with the volume of the traffic flow.

FIGS. 14a and 14b are graphic illustrations showing the procedure of determining a split with a district controller in accordance with traffic volume and density.

FIG. 15 is a graphic illustration showing the procedure for determining a relative offset with a district controller in accordance with the size of the traffic volume.

FIG. 16 is a block diagram showing the construction of a central controller.

FIG. 17 is a wiring diagram showing the construction of a traffic flow detection device.

FIG. 18 is a graphic illustration showing the reference pulse (RP) and the reference frequency signal (RF) which are part of the direction signals sent from a district controller to a local signal controller.

FIG. 19 is a wiring diagram showing the construction of a local signal controller.

FIG. 20 is a wiring diagram showing the construction of the traffic signal receiving device and traffic flow counting device of the district controller shown in FIG. 12.

FIG. 21 is a wiring diagram showing the construction of the cycle determining device shown in FIG. 12.

FIG. 22 is a wiring diagram showing the construction of the offset determining device shown in FIG. 12 in a Type-l district controller.

FIG. 23 is a wiring diagram showing the construction of the split determining device shown in- FIG. 12 in a Type-l district controller.

FIG. 24 is a wiring diagram showing the construction of a central command receiving device shown in FIG. 12.

FIG. 25 is a wiring diagram showing the construction of the offset determining device shown in FIG. 12 in a Type-2 district controller.

FIG. 26 is a wiring diagram showing the construction of a district controller.

FIGS. 27a and 27b are diagrammatic plan views showing two examples of trees for the road network shown in FIG. 1.

Referring to the drawings and first to FIG. 1 thereof, a road network is shown. This road network is divided into a plurality of districts and, first, an optimal control is effected in each district over a plurality of signals located therein. The dot-anddash lines of FIG. 1 represent the border lines of these districts. For example, the district 101 contains two main roads 102 and 103 crossing each other and other auxiliary roads not shown in the figure. What is to be controlled in the district is a plurality of signals installed along the main roads 102 and 103.

The district comprises a mesh of main roads and all the signals installed along these roads are to be controlled.

For the control of signals in each district, one district controller (DC) is installed in each district. For example, DC 104 in FIG. 1 controls the signals on'the main roads 102 and 103 in the district 101. Two types of district controllers are used depending upon the type of district. A district such as district 101, i.e., a district which does not contain any closed loop of main roads, is called a Type-l district, and a Type-l district controller (DCI) is installed in such a district.

A district such as district 105, i.e., a district which contains one or a plurality of closed loops of roads, is called a Type-2 district, and a Type-2 district controller (DCII) is installed in such a district.

All districts are classified as one or the other of the abovementioned two types. One central controller (CC) is installed in the road network as shown in FIG. 1 and controls a plurality of such districts. The central controller governs over all district controllers of such districts to obtain optimal results on the whole network of roads under its control, and has the function of issuing command signals to all district controllers concerning the control parameters.

As a means for directly operating each signal device, a unit signal controller or local controller (LC) is installed at an intersection or a pedestrians way or the like, and operates signal lamps in accordance with the directions from a DC.

The control of signal lamps is always effected by observing traffic flow at that moment. A plurality of trafiic detectors (TD) are provided at suitable locations in the road network and information on traffic flow is sent to district controllers and to the central controller directly or through a district controller.

The relations between the central controller, district controllers (DCI and DCII), local controllers and traffic detectors are shown in FIG. 2.

Before explaining the present system of issuing control directions by way of signal parameters, the definitions of the signal parameters will be given.

A road as shown in FIG. 3 is taken for example. The signal indications at the intersection 301 in the directions 304 and 303 are shown in FIG. 4 at 401 and 402 respectively. In the signal indication 401, g 405 represents the green signal duration, y 406 amber (yellow) signal duration, and r 407 red signal duration. The same applies to the signal indication 402. In this connection, T is a signal cycle length and the ratio of the sum Gil of the green and amber durations to the cycle length, Gall, is called the split in the direction of 304 of FIG.

The signal indications at the intersection 302 of FIG. 3 in the direction 305 are shown in FIG. 4 at 403. As is clear from the Figure, there is a time difference D between the indications 401 and 403. The ratio of this time difference D to the cycle, i.e., D/T, is called the offset of the indications 401 with respect to the indications 403.

If the cycles of all the signals, including signal lamp So of the intersection 302 in a road network are equal, it is possible to determine the ofi'set of every signal in the road network with respect to one indication of signal So. Each signal in the road network has two or more indications, but if the offset for one of them is determined, then the offset for the other indications can be determined. For this reason, one indication is made the main indication for every signal, and the offset of that indication with respect to the main indication of a reference signal is called the absolute offset of that signal.

As two intersections are shown in FIG. 3, the offset of the indication of the signal installed at 301 in the direction 304 with respect to the indication of 302 in the direction 305 is called the relative offset in the direction from 302 to 301. The same definition applies to the opposite direction. An absolute offset is a quantity given or assigned to each signal, while a relative offset is a quantity defined between two adjacent signals.

The above definitions were given with reference to crossroads, but it goes without saying that split and offset may be defined for multiple indications of signals at an intersection even if the intersection is a trifurcate (three-road) or a fiveroad intersection.

The control system of the present invention is characterized in that the signal parameters as defined above, especially the relative offset, are suitably given in accordance with the pattern of trafiic flow in such a way that preference may be given to the traffic which is the main flow in the road network. Furthermore, the system makes it possible to regulate parameters as already mentioned for any given pattern of traffic flowing in whatever type of road network. Thus it makes it possible to effect traffic control of a very high effectiveness which cannot be attained by the systems heretofore employed.

Now the method of determining signal parameters which constitutes the basic principle of the signal control system of the present invention will be explained.

Cycle length: Within one district, one and the same value of cycle is given to all the signals. For this value, one is chosen that can deal with traffic flows at all the intersections in the district without congestion. FIG. 5 is a flow chart for determining the cycle.

First, at each intersection in a district, the maximum values of the traffic volumes on the roads under the control of one and same signal indication are found, and then the sum of these maximum traffic flows is calculated. This sum is called the flow-in trafiic volume of that intersection. For example, consider the intersection 301 in FIG. 3. The larger of the traffic volumes as between the traffic volume in the direction 313 and the traffic volume in the direction 304 is represented by q,,, and the larger of the traffic volumes as between the trafi'tc volume in the direction 303 and the traffic volume in the direction 308 is represented by q,,. Then the sum of q,, and q is the flow-in traffic volume of that intersection.

Then, the flow-in trafiic volumes of all the intersections in the district are compared to find out the intersection which has the largest flow-in traffic volume. This is executed or performed by the block 501 of FIG. 5. The cycle is determined from the values of q,, and q,, for the particular intersection in such a manner as shown in the block 502 of FIG. 5.

In 502, q,,, denotes the maximum traffic volume permissible for the road, and L is what is called the loss time of a signal. Both are values which can be previously determined by observation. It is not necessary to measure flow-in traffic volumes at all intersections. It goes without saying that the number of the intersections where the flow-in traffic may be maximum are limited.

Split: As to the split, a specific value is determined for each intersection respectively by comparing and evaluating the traffic volumes and densities on roads crossing each other at an intersection. If the intersection 301 of FIG. 3 is taken for example, the traffic volumes to be compared are the aforementioned q, and q What is referred to as density is the number of vehicles existing in a unit distance of each of the roads crossing each other. As in the case of q, and q,,, let k, and k represent the maximum values for the traffic density controlled by one and the same indication; then k and k,, are compared and evaluated. The split given to the traffic flow direction of q, (k,,) is represented by q and the split in the q (k,,) direction g from the formulae.

The sum of and g,, is equal to 1. FIG. 6 is a flow chart for the determination of splits g and g at one intersection.

First, it is checked in block 610 whether k,, or k,, is greater than a given value k,,. If either of k,, and k is greater than k,,, splits g,, and g, are determined as shown in the block 602.

Here, what is done by block 602 is to determine the splits so as to make the ratio of g zg equal to k,,:k,,. By determining the values in this manner, congestion can be resolved as quickly as is possible.

When both k,, and k,, are smaller than k,,, q, and q,, are then measured and the values of q,, and q,, are compared as shown in the block 604. If q,, is greater than q,,, q and q are determined as shown in the block 605.

By determining splits in this way, it is possible to eliminate congestion and to minimize the total waiting time.

If q, is equal to q,, at 604, q and q,, are given equal values, namely, 0.5, at the block 607. In this case, too, the result is the same as that in the case of 605.

Ifq is greater than q at 604, splits q, and q are determined as shown at 606.

The density should be measured at intersections in the road network where there is the possibility of the density exceeding k,,.

Offset: Offset is given to the road section between two adjacent signals (hereinafter referred to'as a road section). A relative offset is determined for some of the road sections which will be explained later. The offset is given a value that minimizes the total waiting time of both traffic flows in accordance with the conditions of trafiic flows traveling a road section in both directions. There are two kinds of relative offset adopted for minimizing waiting time preferential offset and balanced offset. Explanation will be made with reference to the intersections 301 and 302 of FIG. 3 for example.

Now let us consider the traffic flow of this road section in the direction of 309. If a relative offset is determined in the direction 309 in such a manner that the traffic flow starting from the intersection 302 at any time during the green duration and traveling at a prescribed speed reaches the intersection 301 when 301 has a green signal to let the flow pass the intersection 301 without stopping in the direction 309, then this offset is called a preferential offset in the direction 309. Similarly, an offset determined to let the traffic flow in the direction 310 pass 302 without stopping is called a preferential offset in the direction 310.

Generally speaking, it is impossible to give a preferential offset to both directions 309 and 310. The question of what offset will be given to the other when a preferential offset is given to one direction is determined by the length of the road section, traveling speed of vehicles and the cycle. As is clearly seen from FIG. 4, the preferential offset is an offset making D of that figure equal to the traveling time of vehicles for the distance, and the value of the offset is equal to the ratio of the above-mentioned traveling time to the cycle.

A balanced offset is such that the value of D of FIG. 4 is made zero or one half of the cycle.

How to determine a relative offset which minimizes the waiting time in one road section is shown in FIG. 7.

FIG. 7 is a flow chart for determining a relative offset for a road section as shown in FIG. 3. q and q represent the traffic volumes in the directions 309 and 310 respectively. a and a are factors determined by the time waveform of the traffic flows and take a value between and .l. x is the relative offset in the direction 309, and u the fractional part of the quotient obtained by dividing the traveling time by the cycle, the value being between 0 and I.

First at the block 701, it is determined whether the maximum value of the traffic volumes in both directions is not in excess of a fixed value q q is a constant dependent on the road, and is a value about one-tenth of the maximum traffic volume of the road. If both a and q are smaller than q then it is'determined at the block 702 whether the value u is between one-fourth and three-fourths. If it is within this range, the value of the relative offset is determined to be one-half. Ifu is less than one-fourth or greater than three-fourths, the offset is decided to be 0.

If either of q and q is found greater than q, at the block 701 then a x q and a x q are compared at 706. a q and a q are called the effective traffic volume of the two directions respectively. a and a are constants dependent on the roads and are parametersrepresenting the controlling effect of the ofiset.

They are so determined that if a q is greater than or equal to a q, the offset value x becomes 14, and if a q is smaller than a q', the offset value becomes 1 u.

However, it is only a part of the road network where a relative offset can be determined in the above-mentioned manner. Explanation will be made with reference to the district 105 of FIG. 1 for example.

Now, if the respective relative offsets of the road sections 110, 111 and 112 shown in FIG. 1 are determined, then the time lags between the signals 106, 107, 108 and 109 shown in FIG. 1 are determined. In consequence, the relative ofiset for the road section 113 shown in FIG. 1 is also determined. In other words, the offset for one of the road sections forming a closed loop of road network is determined when the offsets for all the other road sections have been determined. Consequently, it is impossible to apply the offset control shown in FIG. 7 to that one section.

Next, we will discuss road sections to which relative offsets can be given in a road network in general. A road section to which a specific relative offset can be given is limited to those of a road network which forms a tree not containing any closed road loop. What is calleda tree is a figure obtained as a result of eliminating certain road sections of a road network to eliminate closed road loops, all the intersections of the original road network being retained adjacent to each other as before. Examples of trees for the road network of district 105 of FIG. 1 are shown in FIG. 8a and b. There may be many trees for this road network other than those shown in FIG. 8, and it is possible to determine both preferential and balanced offsets for any and all road sections forming any one of such trees.

The question of what shape tree should be given an offset is of great importance, and the way to solve this question is an outstanding characteristic of the present invention.

Now, therefore, we will define a value 1:, which is called the offset efi'ectiveness value. E is a value defined for each of the roadsections in the road network. What is meant by E is the difference between the average value of waiting time of traffic flows in both directions for the value of relative offset from 0 to l and the waiting time of traffic flows in both directions when a preferential offset is given in the manner of FIG. 7. In other words, it is the amount of improvement obtained in waiting time when a preferential offset is given to that road section, or the difference between the average and minimum values of traffic delay time in both directions.

E is a quantity determined by traffic volumes in both directions, time required for traveling the road section and cycle length. i

How the shape of a tree is determined is shown in FIG. 9.

First at the block 901 the values of E for all the road sections are calculated and the road section having the greatest E is chosen at the block 902. Then, it is determined at the block 903 whether a closed loop of roads is formed or not if this road section is addedto the group of road sections already chosen. If not, this road section is added as a section of the tree, as shown at 904. If a closed loop of roads is formed at 903, this road section is eliminated from the original network and a search for the road section having the next greatest E is made. The same procedure is also applied after the block 904 operation.

' The procedure is successively executed until all the road sections have been checked in this way, and a complete tree is obtained as a result of 904. If a relative offset is determined for this tree by the method shown in FIG. 7, the waiting time can be made as small as possible.

The tree obtained in the above-mentioned way may be a complete tree which covers the whole district or may be one or a plurality of partial trees covering a part of the district.

The above-mentioned ways of selecting split and offset are example embodiments of the present invention.

As already mentioned, the control system consists of a central controller (CC), district controllers (DC), local controllers (LC) and traffic flow detectors (TD), as shown in FIG. 2. The district controllers may be divided into two classes, Type- 1 district controllers (DCI) and Type-2 district controllers (D- CII). A central controller is a device to effect an overall control of signals, one central control being installed for one road network to be controlled. As shown in FIG/1, a road network is divided into a plurality of districts. The districts are divided into two classes, those which do not contain any closed loops of main roads (for example, 101), and those which contain such loops (for example, 105). Type-l district controllers are installed in the former and Type-2 district controllers are installed in the latter.

1. Traffic flow detector (TD): The construction of a trafiic detector is shown in FIG.- 10. The vehicle detector 1001 is installed on the road and sends out a detection signal when a vehicle passes it. This signal is sent to the district controller via the transmission line 1003 from the detection signal transmitter circuit 1002.

2. Local controller (LC): A local controller is a device employed to operate a plurality of signal lamps installed at one intersection. Its construction is shown in FIG. 11. The control signal sent from the district controller via the transmission line 1101 is received by the signal-receiving circuit 1102 and is sent on to the switching signal generating circuit 1105. At switching signal generating circuit 1105, an electric signal indicating the on and off time for the signal lamps is obtained in accordance with the command received from the district controller, and this signal is sent to the switching circuit 1107 to switch the signal lamps on and off accordingly.

3. District controller (DC): A district controller determines signal parameters, i.e., cycle length, split and offset, in the ways described in connection with FIGS. 5, 6, 7 and 9 on the basis of information on traffic flows for the signals in the district where the controller is installed, and issues commands to the local controllers in accordance with the results thus determined. The construction of a district controller is shown in FIG. 12, which gives an example embodiment. The information on traffic flow sent from the trafiic detector via a transmission line group 1003 is received by the traffic information receiving device 1201, and is then sent to the trafiic counting device 1202, where traffic volume and density of each road section are computed.

. A cycle length selector 1205 is a device to determine the cycle length in the manner as described in conjunction with FIG. 5. It determines the cycle length in accordance with traffic volume at the intersection sent from 1202. The value of the cycle length is determined in the following way: As shown at the block 501 in FIG. 5, first the maximum flow-in traffic volume is obtained. Then the value of the cycle length is determined at 502. In the present system, several values are previously prepared for the cycle to be given at the block 502, and one of those values is selected in accordance with the traffic flow.

In FIG. 13, it is shown how to select a cycle length. The q, q, on the abscissa is the q,, q,, shown at the block 502 in FIG. 5, and the cycle is determined depending upon what region of FIG. 13 this value is in. For example, if q q, is in the region 1302, then a cycle is given by substituting the q,, q, in the formula for T at the block 502 in FIG. with q shown at 1302 in FIG. 13. The same applies to other regions.

The split selecting device 1206 shown in FIG. 12 is a device for determining the split of each of the signals in the district controlled by the district controller in the manner shown in FIG. 6. Traffic volume and density at each intersection is sent from 1202 to 1206. As in the case of the cycle, several values of the split are previously prepared, and one of them is selected in accordance with the trafiic volume and density. The method of this selection is shown in FIG. 14.

The judgment shown at the block 601 in FIG. 6 is made as shown in FIG. 14. That is to say, it is judged whether both the respective densities k, and k of traffic flows crossing each other do not exceed a certain value. It is seen that if both k and k are in the region 1401, both do not exceed the fixed value, and that if they are in regions other than 1401, either It or k,, exceeds the fixed value. In case the fixed value is exceeded, the regions 1402, 1403, 1404, 1405 and 1406 are provided in 'such a manner that the value of split shown at the block 602 in FIG. 6 may be given, and one split value is provided for each of them. In case k,, and k, are in the region 1401, q and q, are compared by the graph 0 shown in FIG. 14, and the values of split are prepared in accordance with the regions 1407 1413. At graph a, the region 1407 corresponds to the block 607 of FIG. 6, the regions 1408 1410 correspond to the block 606 of FIG. 6, and the regions 1411 1413 correspond to the block 605, and each of the blocks gives the value of split determined.

The offset selecting device 1207 of FIG. 12 has the function of determining the relative offset for the road section of the road network under the control of the district controller and then converting it into the absolute offset for each signal.

The way in which the offset is determined is as follows: As already stated, first the optimal tree of the road network is selected by the method shown in FIG. 9. Then, a relative offset is given to the road section making up this tree by the method shown in FIG. 7. The offset selecting device has some difference in its function according to whether it is for a Type-l district controller or a Type-2 district controller.

In the case of a Type-l district controller (DCI), no closed road loop is included in the road network under its control as shown at 101 in FIG. 1. The road network itself is therefore a tree, and it is not necessary to select a tree. With DCI, therefore, a device which realizes the procedure of FIG. 7 is good enough. This is done in such a manner as shown in FIG. 15. The abscissa and ordinate of the graph shown in FIG. 15 represent the effective traffic volumes in the up and down directions of a road section respectively. The region 1501 corresponds to the blocks 703 and 704 of FIG. 7, and if q a, q a are in this region, an offset of 0 or one-half is given. Which of these two values should be given is determined by the length of the road section, and either of the values is provided for each road section. The regions 1502 and 1503 correspond to the blocks 708 and 709 of FIG. 7 respectively, and are also provided with a value of relative offset previously determined.

In the case of DCII, the function of selecting a tree is required, because the road network contains a closed loop of road sections such as district 105 of FIG. 1 does. For the selection of a tree, there are the following two types of procedures.

The first of them is as follows: Optimal trees of several types are previously prepared for several types of traffic flow patterns which are likely to take place. One of these is selected in accordance with the condition of traffic flows. The trees to be prepared are previously obtained by the procedure shown in FIG. 9. This procedure is highly useful for a district which has only a few patterns of traffic flow.

The second of the procedures is one in which a device for.

determining a single tree by the method of FIG. 9 is provided. This is useful where the road network has many possible patterns of trafiic flow.

Both procedures make it possible to determine a tree of the shape suitable for the traffic flows at the moment and make it possible to enhance the controlling efi 'ect of an offset. This is one of the characteristics of the present invention, which the conventional systems do not possess.

When a tree has been decided on, a relative offset is determined by the procedure of FIG. 7 just as in the case of DCI.

The cycle, split and offset which have been determined by the cycle selecting device 1205, split selecting device 1206 and offset selecting device 1207 respectively are then sent to the signal parameter sending device 1209 and are sent to each local controller accordingly. The signal sending device has a time buffer function in changing cycle, split, and offset to prevent transient confusion of traffic flow.

The information on traffic volume and density, counted by the traffic counter, is sent to the central controller from the traffic parameter sending device 1203 of a district controller.

On the other hand, the central command receiving device 1204 of district controller receives control commands from the central controller, and sends this information to cycle length selecting device 1205. If control commands are received from the central controller, device 1205 determines the value of cycle in accordance with the cycle command sent from the central command receiving device 1204 and the offset command, and also adjusts the absolute offset of the basic signal which becomes the basis for absolute offset in the district controlled. As a result, it becomes possible to specify the offsets of signals in adjacent districts, for example districts 101 and in FIG.- 1, and to adjust the relative offset of the road section 116 which comprises the border line between the two districts.

4. Central controller (CC): The central controller specifies to each district controller signal parameters to be set. Of the parameters specified, the offset is of the greatest importance. As already mentioned, it is possible to set signal parameters individually for each DCI or DCII installed in each district, and the central controller makes it possible to adjust these individually determined parameters to coordinate the districts and to effect control over one area consisting of an equivalent combination of the districts. Concretely speaking, central controller (CC) controls the parameters of each of the districts controlled by DCI of the road network shown in FIG. 1 and forms one or a plurality of trees covering the whole or part of the road network. A plurality of DCI belonging to one and the same tree function as is they were one district controller and controls with a very high efficiency, vehicles traveling in the area. In addition, it is possible, as already mentioned, to alter the form of the tree determined in accordance with the pattern of traffic flow. A highly flexible control can thus be effected.

An'example of the construction of a central controller em bodying the invention is shown in FIG. 16. The transmission line 1607 is from the traffic flow parameter sending device 1203 of FIG. 12, and information on traffic volume is sent to the trafiic flow pattern selecting device 1601 from district controller via 1607. The traffic fiow pattern selecting device 1601 judges to which of the previously provided patterns the trafiic flow at the moment corresponds, and the result of the judgment is sent to the combination pattern memory device 1602. The combination pattern memory device is a device which memorizes how district controllers should be combined and what signal parameters should be given in accordance with the traffic flow pattern selected out. The procedure for combining them is as follows: For each of the various traffic flow patterns of the different districts, a road network tree is obtained as previously described, and a combination is made of them to form a resultant tree. When it has been decided how to combine them, this information is sent to the signal parameter generating device 1603 to generate signal parameters which make the combination of the adjacent districts possible.

On the other hand, the traffic flow pattern indicating device 1605 always indicates the condition of traffic flows in the road network. Especially, when an abnormal condition of traffic flows has taken place, the operator can read the indication and order a specific pattern through the combination pattern setting device 1606. The result is sent to the signal parameter generating device 1603 and the information from combination pattern memory device 1602 is disregarded.

Signal parameters generated at device 1603 are sent to the signal parameter sending device 1604 and then passed on to each district controller via the transmission line 1608.

A central controller may be given the functions of not only specifying cycle and relative offset for districts but also of specifying offset and split within each individual district.

Now we will explain the control system of the present invention, describing an example of the control device for carrying out the afore-mentioned control procedures.

1. Traffic Detector (TD) An example of a practical construction of a traffic detector is shown in FIG. 17.

The loop wire 1701 buried under the surface of the ground is connected to the input terminal of the oscillator 1702. The oscillator 1702 is a so-called LC oscillator, so that the inductance of loop 1701 changes when a vehicle passes above the loop 1701. A change in inductance causes a change in the oscillation frequency of oscillator 1702.

The signal oscillated by oscillator 1702 is sent to the frequency discriminating circuit 1703, and the change in frequency is transferred as a voltage signal at the output terminal of the frequency discriminating circuit 1703. Thus, the said voltage signal is given to modulator 1704 through the line 1710a each time a vehicle passes above the loop 1701.

Similar detecting signals are sent to other input terminals of the modulator 1704 through the lines 1710 b and 1710c from other frequency discriminating circuits. Such a plurality of input signals are modulated, miltiplexed and sent to a district control through the cable 1707.

For the traffic detector, vehicle detectors of the known type, which utilize the phenomenon of change caused by a vehicle in electrostatic capacity, pressure, mechanical position, supersonic wave, electromagnetic wave, etc., may also be used.

2. Local controller (LC) in FIG.,18 are shown the control signal sent from the district controller to the local controller and the signal light operating pulse obtained at the local controller on the basis of said signal.

The repetitive pulse 1801 has a cycle of period T equal to the signal light cycle, and the repetitive pulse 1801 corresponds to the starting point of the indication of one reference signal chosen as basic signal in a district. It will hereinafter be referred to as the reference pulse (RP).

The alternating current signal 1802 has a frequency which is p. times the frequency of the reference pulse, for example, l,000 times the frequency of said reference pulse RP, and will hereinafter be referred to as the reference frequency signal (RF).

There are also the direct current signal VS which shows the value of split and the direct current signal V which shows the value of offset. Together with signals RP and RF, these are sent from the district controller to the local controller by a modulation means suitable for the transmission line.

An example of the construction of the local controller is shown in FIG. 19.

The control signals sent from the district controller via the cable 1900 are received and selected by the signal receiving device 1901 and RP, RF, V0 and VS signals are obtained at its output terminals 1902, 1903, 1904 and 1905 respectively. Since 1901 can be made by publicly known techniques, we will not explain it here in detail.

The timing circuit 1909 is a circuit to generate the absolute ofiset. If the signal RP obtained at the terminal 1902 is applied to the flip-flop 1906, the flip-flop 1906 is set and the gate 1907 opens. As a result, signal RF appearing at 1903 is added to the input terminal 1910 of the accumulating counter 1908. The accumulating counter 1908 has a negative electric charge stored in the condenser 1911 for every cycle of signal RF applied to said input terminal 1910 and the output voltage of the field effect transistor 1913 increases. This voltage is applied commonly to the A-terminals ofthe compares (CM?) 1918 and 1919. Signal V0 appearing at the terminal 1904, is applied to the B-terminal 1917 of the CMP 1919, and at the same time applied to the B-terminal of CMP 1918 after subtraction of the specified basic voltage at the subtractor 1920. CMP is a circuit which generates an output in case input voltage at the A-terminal is greater than the input voltage at the 8- terminal. Now, if voltage at the A-terminals of CMP 1914 and 1916 rises, it first exceeds the voltage of the B-terminal of CMP 1915 and then exceeds the voltage of the other B-terminal of CMP 1917. In consequence, an output appears at CMP 1918 and then CMP 1919, and as a result, the monostable multi-vibrators 1921 and 1922 cause a pulse of a fixed width having a prescribed time interval to be obtained at the output terminals 1923 and 1924. The pulse obtained at 1924 is a pulse delayed from signal RP by the timing specified by signal V0; that is to say, it corresponds to the pulse 1804 of FIG. 18, representing the start point of the green signal indication in the main direction. The pulse obtained from the terminal 1923 corresponds to the pulse 1803 of FIG. 18, and is a pulse preceding the pulse 1804 by a prescribed timing and represents the starting point of the amber in the secondary direction.

When an output appears at the terminal of CMP 1919, 1908 is reset via the diode 1912, FF 1906 is reset at the same time, the gate 1907 is closed and counting is not done until the next signal RP appears at the terminal 1902.

The timing circuit 1925 is of the same construction as 1909. By the pulse 1804 representing the start point of the green signal given to the terminal 1924 and signals RF and VS obtained at the terminals 1903 and 1905 respectively are obtained the pulse representing the end of the green signal in the main direction and the end of the amber direction at the respective output terminals 1926 and 1927. These correspond to the pulses 1805 and 1806 shown in FIG. 18 respectively.

The switching circuit 1928 shown by a dashed line is a circuit which converts the pulses obtained as mentioned above for switching time of the signal lights into signals for actually operating the signal lights. The output terminals 1935, 1936 and 1937 issue signals to light the green, amber and red signals in the main direction respectively, and the output terminals 1938, 1939 and 1940 issue signals for lighting the green, amber and red signals in the secondary direction respectively.

When the electric power source is first switched in, a reset signal is applied to the input terminal 1934 and FF 1929 and FF 1931 are reset and FF 1930 is set with output signals appearing at terminals 1937 and 1939 for red in the main direction and yellow in the secondary direction. Even when a pulse corresponding to 1803 of FIG. 18 is then obtained from 1923, the condition remains unchanged. Then, when a pulse corresponding to 1804 is obtained from the terminal 192 4, FF 1929 and FF 1931 are set and an output appears at the terminals 1935 and 1940, when the main direction becomes green and the secondary direction red. In a like manner, it will be clearly seen that lighting signals for correct signal indications are obtained at the terminals 1935 1940 as pulses ap pear in repetition at the terminals 1926, 1927, 1923 and 1924.

These outputs are then sent to the light switching circuit 194] to switch the signal lights on and 011'.

3. Type-1 District Controller DCI) The Type-l district controller, whose construction is shown in FIG. 12, will be explained with reference to an example.

An example of the traffic flow signal receiving device 1201 and of the traffic flow counting device 1202 shown in FIG. 12 are shown in FIG. 20.

Traffic flow information sent from the trafiic detector via the cable 2000 is received, selected and detected by the signal receiving device 2002 enclosed by a dashed line and is obtained as a voltage signal at the tenninals 2011 2012.

The outputs of the terminals 2011 2012 correspond to the outputs of the detectors 2004, one to one, and a voltage signal is obtained at each terminal while a vehicle is present above the loop. The output of 2011 is then sent to the counting circuit 2005 enclosed by a dashed line. This signal is differentiated and is sent to the monostable multivibrator (MM) 2007 and at the same time to FF 2008. For the output of (MM) 2007, a pulse of a fixed time width is obtained every time a vehicle is detected, and for the output of (FF) 2008, a pulse having a time width equal to the time required by the vehicle to pass over the detector is obtained. These are then sent to the integrating circuits 2009 and 2010 and averaged, voltage signals showing traffic volume and density respectively are obtained at its output terminals 2015 and 2016. Counting circuits of the same construction as 2005 are provided in parallel, one corresponding to each detector, and count their respective traffic volume and density. These values are used for the determination of cycle, split and offset. The required number of the same signal receiving devices 2002 followed by counting circuit 2005 as explained above are provided in parallel.

The construction of the cycle selecting device 1205 shown in FIG. 12 is shown in FIG. 21.

The input terminals 2101 and 2102 respectively, receive the traffic volumes in two directions at one intersection, for example, the direction 303 and the direction 308 at the intersection 301 in FIG. 3. In a like manner, signals corresponding to the traffic volumes in the direction 304 and the direction 313 are applied to the terminals 2103 and 2104. The circuit 2105 is a maximum value extracting circuit (MAX), and the maximum value of the voltage applied to the terminals 2101 and 2102 is obtained for its output. The circuit 2106 is also a MAX, and their output voltages are added then at the adding circuit 2107. The adding circuit can be made by the usual technique. Similar maximum value extracting circuits and adding circuits are installed in parallel in a number equal to the number of principle intersections, and their outputs are sent to the input terminal of MAX 2108, the maximum flow-in traffic volume at each intersection of the route being obtained at the output terminal 2133 as a result. Then this output is commonly fed to the input terminals A of the comparing circuits 2109 2114.

A suitable voltage is fed to the other input terminal B of each comparing circuit (CMP) by the level setting circuit 2134. The voltage at the input terminal of each CMP is provided to give a critical value corresponding to q q q of FIG. 13. In the present example where N 7, a voltage providing q is applied to the input terminal B of 21 14, a voltage giving q; to the input terminal B of2133, and so forth until a voltage corresponding to q is provided to the input terminal B of 2109.

Now, if the traffic volume is less than q there will be no outputs from the comparing circuits 2109 2114. This condition is made O." Conversely, the condition producing 1 is made Because of this, all the gates 2115 2119 are closed, so that no output appears at the terminals 2126 2131, either. On the other hand, as the output of CMP 2114 is inverted by the inverter 2125 and l is obtained, an output is obtained at the terminal 2132.

If the traffic volume is greater than q, and smaller than q the output of CMPs 2109 2113 is O and only the output of 2114 is ,l. Consequently, the terminals 2126 2130 are 0.As the input of the inverter 2125 is 1, its output, i.e., the terminal 2132, becomes 0. On the other hand, as the input of the inverter 2124 is 0, its output becomes Also, as the output of CMP 2114 is 1," the gate 2119 opens and an output 1 is obtained at the terminal 2131. Likewise, if the traffic volume becomes greater, an output is obtained at one of the terminals 2130 2126, and it is judged according to the traffic volume to what region shown in FIG. 13 it belongs. Needless to say, the terminals 2132, 2131, 2130. 2126 correspond to the regions 1301, 1302 1304 of FIG. 13 respectively.

The output from the terminals 2126 2132 is sent to the level setting gate circuit 2139 enclosed by a dashed line. The level setting gate circuit is a circuit which generates a voltage signal representing a cycle length in accordance with the input signal showing the judgement result.

The magnitude of voltage for each cycle is previously calculated by the procedure shown in FIG. 5, and is set at the level setting circuits (LS) 2140 2146. The output voltage of LS 2140 2146 is then sent to the variable frequency oscillating circuit 2148 via the mixing circuit 2147. The circuit 2147 is quite the same construction as the maximum value extracting circuit 2100, and the circuit 2148 is a circuit issuing alternating current signal having a cycle corresponding to the input voltage. The output of the circuit 2148 then enters the frequency step down circuit 2149, and a pulse stepped down to a frequency of l/p., for example one one-thousandth, is obtained for the output of the circuit 2149. This pulse is the reference pulse representing the cycle and the output frequen cy of the circuit 2148 is used for the reference cycle signal.

As described above, a cycle determination explained in FIG. 5 and FIG. 13 is executed by the apparatus of FIG. 21 in accordance with the traffic flows in the road network.

When command information on cycle and absolute offset are received from the central controller, the signals RP and RF determined by the above-mentioned procedure are ignored.

For this purpose, gates 2160 2165 are provided, and when a signal CR representing command information is given to the terminal 2166, the output signals of 2148 and 2149 are ignored at 2161 2164 and the reference pulse (RFC) and reference frequency (RFC) sent from the central controller are obtained at the output terminals via the gates 2162 and 2165.

As the absolute offset in the system is represented by a time lag from the signal RP, the absolute offset for the whole system is regulated by sending out RPC in place of the signal RP as mentioned above. 1

As in the case where the road network under the control of a DCI consists of main roads extending radially as in the district 101 in FIG. 1, the traffic volume in the road sections in the district are generally considered to be almost equal to each other. Consequently, when a preferential offset is given, it is possible to give a common direction to the road sections belonging to the main road. In the present example, unit offset selecting devices are provided at the rate of one for each main road extending radially. An example of the unit offset selecting device 1207 shown in FIG. 12 is given in FIG. 22. The unit offset selecting circuits 2201, 2202, 2203 and 2204 are devices to choose one of the preferential offsets and the equal offset by the procedure shown in FIG. 15.

Terminals 2211 and 2212 respectively, receive the traffic volumes q and q in both directions of a main road in terms of a voltage, and fixed ratios at and a are calculated at the level adjusters 2219 and 2220 respectively. These ratios are the a and a representing the offset control effect shown in FIG. 7 and FIG. 15, which are previously determined in accordance with the road. Then these values are sent to the maximum value extracting circuit 2221 and CMP 2224. The output of 2221 is compared at CMP 2222 with the output of the comparing voltage generating circuit 2223 which gives a basic value for either of a q or a q. This comparing voltage establishes the borderlines 1504 and 1505 in FIG. 15, and thereby the circuit judges whether a q and a q are within the region 1501 of FIG. 15 or not.

CMP 2224 is a circuit to compare the magnitudes ofa q and a q. Where a q is greater than a q, l is produced at the output terminal and the border 1506 of FIG. 15 is given. These results are synthesized by combinations of the inverters 2227 and 2228 and AND-gates 2225 and 2226, and the result of the judgment is obtained at the terminals 2229 2231. The terminals 2229, 2230 and 2231 have an output when a q and a q are in the regions 1503, 1502 and 1501 of FIG. 15 respectively. 

1. A traffic signal control system for a district having a complex road network pattern, including a plurality of traffic detectors each output of which is processed by traffic counting means for producing information indicative of traffic flow in a selected road network, cycle selecting means responsive to said information to select an optimal traffic signal cycle length for the road network, and split selecting means responsive to said information to determine the traffic signal split value of each intersection in the road network, characterized by offset selecting means for determining an optimal tree pattern of said road network in response to said traffic flow information and assigning an optimal offset to each road section included in said tree.
 2. The traffic signal control system in accordance with claim 1, wherein said offset selecting means determines the optimal tree on the basis of on-line computing by a gate network which has the same pattern as the road network and assigns the preferential or balanced offset to each road section included in the determined tree.
 3. The traffic signal control system in accordance with claim 1, wherein said offset selecting means selects the optimal tree from a plurality of tree patterns predetermined in accordance with the standardized traffic patterns of the road network and assigns the preferential or balanced offset to each road section included in the selected tree.
 4. A traffic control system for a large scale area which is divided into a plurality of districts each of which is controlled by a district controller in accordance with claim 1 characterized in that the control system includes a central controller including a traffic pattern detector which receives the traffic information transmitted from the district controllers to determine the traffic flow pattern of the area and a switching network which, responding to the flow pattern, selects at least one tree pattern of the area fRom predetermined sets of tree patterns and coordinates the district controllers of each district belonging to each selected tree pattern by assigning a cycle value common to each group of districts included in a tree pattern and assigning optimal offset values to road sections which connect the adjacent districts.
 5. A road traffic control system for the regulation of vehicular traffic flow in a district having a plurality of joined road sections with each having signalized intersections at both ends, comprising traffic flow detection means operable to produce traffic flow information representative of the traffic flow on each road section; a district controller device connected to receive said traffic flow information for the determination therefrom of cooperative values of the cycle, split and relative offset for each traffic signal at said signalized intersection to provide optimal traffic flow in said district; said district controller including a traffic counting device operable to determine and produce from said traffic flow information traffic volume and density output information for each of said road sections, a cycle selection circuit operable to determine and produce compatible cycle duration information with the use of said output information for each traffic control signal at said intersections, a split determination circuit operable to determine and produce compatible split information with the use of said output information for each traffic control signal at said intersections, and an offset determination circuit operable to determine, produce and assign optimal offset value information with the use of said output information for each traffic control signal at said intersections; local controller means connected to receive said cycle, split and offset information and operatively connected to said traffic control signals to cooperatively actuate them in accordance with said cycle, split and offset information to provide optimal traffic flow in said district; said offset determination circuit including an offset effect circuit receiving said output information and operable to calculate and produce therewith offset effect quantum information representative of the offset effect for each road section, tree selection circuit means connected to receive said offset effect quantum information and operable to successively select the offset effect information having the largest value and rejecting offset effect information values which represent road sections that would form a closed loop with those road sections of previously selected higher offset effect signal values until all of said road sections have been accordingly covered such that a connected road section optimal tree pattern without closed loops is selected, and an offset selection circuit connected to said tree selection circuit to select and assign said cooperative optimal offset information for each road section included in said tree.
 6. The road traffic control system of claim 5 wherein said tree selection circuit comprises a gate network which has the same pattern as the road network and selects said tree by on-line computing.
 7. The road traffic control system of claim 5 wherein said tree selection circuit comprises means to select the optimal tree from a plurality of tree patterns predetermined in accordance with the standardized traffic patterns of the road network.
 8. The road traffic control system of claim 5 which includes a large traffic control area for control which is divided into a plurality of districts each of which is controlled by a district controller in accordance with claim 5, characterized in that the control system includes a central controller including a traffic pattern detector which receives said output information transmitted from the district controllers to determine the traffic flow pattern of the area and a switching network which, responding to the flow pattern, selects at least one tree pattern of the area from predetermined sets of tree patterns and coordinates The district controllers of each district belonging to each selected tree pattern by assigning a cycle value common to each group of districts included in a tree pattern and assigning optimal offset values to road sections which connect adjacent districts.
 9. In a road traffic control system for the regulation of vehicular traffic flow in a district having a plurality of road sections with each having signalized intersections at both ends, a traffic signal offset determination circuit comprising, offset effect circuit means to calculate the values of offset effect for all of said road sections, tree selection circuit means connected to receive the output of said offset effect circuit means to successively select the road sections having the largest offset effect value and reject those road sections which would form a closed loop with previously selected road sections of higher offset effect value until all of said road sections have been accordingly considered such that a connected optimal road section tree pattern is selected without closed loops, offset selection circuit means connected to said tree selection circuit means to produce and assign cooperative optimal offset values to the traffic signals controlling the road sections forming said tree such that the vehicular travel delay time therealong in both directions is minimized. 